The surface-treated steel sheet conventionally used as a can material is predominately a tin-plated steel sheet such as tinplate, LTS and TNS, a nickel-plated steel sheet (TFS-NT), or a electrolytically chromium-plated steel sheet (TFS-CT). Usually, a chemical conversion treatment is applied to the plated surface of such a steel sheet and the adhesion to a lacquer or a resin film is thereby secured.
The chemical conversion treatment of currently commercialized surface-treated steel sheets for cans is in most cases a dip treatment or cathodic electrolysis treatment using an aqueous solution mainly comprising bichromate or chromic acid. By way of exception, there is known a cathodic and anodic electrolysis treatment of tinplate in an aqueous phosphate solution disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 52-68832 and 52-75626, but the application thereof is limited to a milk powder can used with its inner surface remaining uncoated. The main reason why this cathodic and anodic electrolysis treatment is not applied to a beverage can or a food can except for a milk powder can is because the adhesion of a lacquer or an organic film such as resin film is insufficient.
On the other hand, a chromium(III) oxide film obtained by a dip treatment or cathodic electrolysis treatment using an aqueous solution mainly comprising bichromate or chromic acid has a great effect of enhancing the adhesion of an organic film, and although various treatments as an alternative to this chemical conversion treatment are being studied, these are not put into practice at present. For example, Japanese Unexamined Patent Publication (Kokai) No. 59-47396 discloses an electrolytic tinplate for DI cans, on which a phosphoric acid-based film is formed by a dip treatment, and Japanese Unexamined Patent Publication (Kokai) No. 52-92837 discloses a method of performing an anodic treatment in a phytic acid or phytate solution.
In recent years, many techniques of applying a film to a tin plating layer by using a silane coupling agent are disclosed. For example, Japanese Unexamined Patent Publication (Kokai) No. 2002-285354 discloses a steel sheet and a can, where a silane coupling agent coating layer is provided on an Sn layer or Fe—Sn alloy layer of a tin-plated steel sheet, and Japanese Unexamined Patent Publication (Kokai) No. 2001-316851 discloses a tin-plated steel sheet where on a tin plating layer, a chemical conversion film containing P and Sn is formed as a lower layer and a silane coupling layer is formed as an upper layer. Furthermore, techniques similar to Japanese Unexamined Patent Publication (Kokai) No. 2001-316851 are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 2002-275643, 2002-206191, 2002-275657, 2002-339081, 2003-3281, 2003-175564, 2003-183853, 2003-239084, 2003-253466 and 2004-68063.